From: Martin KaFai Lau kafai@fb.com
commit f30d4968e9aee737e174fc97942af46cfb49b484 upstream.
Below is a simplified case from a report in bcc [0]:
r4 = 20 *(u32 *)(r10 -4) = r4 *(u32 *)(r10 -8) = r4 /* r4 state is tracked */ r4 = *(u64 *)(r10 -8) /* Read more than the tracked 32bit scalar. * verifier rejects as 'corrupted spill memory'. */
After commit 354e8f1970f8 ("bpf: Support <8-byte scalar spill and refill"), the 8-byte aligned 32bit spill is also tracked by the verifier and the register state is stored.
However, if 8 bytes are read from the stack instead of the tracked 4 byte scalar, then verifier currently rejects the program as "corrupted spill memory". This patch fixes this case by allowing it to read but marks the register as unknown.
Also note that, if the prog is trying to corrupt/leak an earlier spilled pointer by spilling another <8 bytes register on top, this has already been rejected in the check_stack_write_fixed_off().
[0] https://github.com/iovisor/bcc/pull/3683
Fixes: 354e8f1970f8 ("bpf: Support <8-byte scalar spill and refill") Reported-by: Hengqi Chen hengqi.chen@gmail.com Reported-by: Yonghong Song yhs@gmail.com Signed-off-by: Martin KaFai Lau kafai@fb.com Signed-off-by: Daniel Borkmann daniel@iogearbox.net Tested-by: Hengqi Chen hengqi.chen@gmail.com Acked-by: Yonghong Song yhs@fb.com Link: https://lore.kernel.org/bpf/20211102064535.316018-1-kafai@fb.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/bpf/verifier.c | 18 ++++++------------ 1 file changed, 6 insertions(+), 12 deletions(-)
--- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -2930,9 +2930,12 @@ static int check_stack_read_fixed_off(st reg = ®_state->stack[spi].spilled_ptr;
if (is_spilled_reg(®_state->stack[spi])) { - if (size != BPF_REG_SIZE) { - u8 scalar_size = 0; + u8 spill_size = 1;
+ for (i = BPF_REG_SIZE - 1; i > 0 && stype[i - 1] == STACK_SPILL; i--) + spill_size++; + + if (size != BPF_REG_SIZE || spill_size != BPF_REG_SIZE) { if (reg->type != SCALAR_VALUE) { verbose_linfo(env, env->insn_idx, "; "); verbose(env, "invalid size of register fill\n"); @@ -2943,10 +2946,7 @@ static int check_stack_read_fixed_off(st if (dst_regno < 0) return 0;
- for (i = BPF_REG_SIZE; i > 0 && stype[i - 1] == STACK_SPILL; i--) - scalar_size++; - - if (!(off % BPF_REG_SIZE) && size == scalar_size) { + if (!(off % BPF_REG_SIZE) && size == spill_size) { /* The earlier check_reg_arg() has decided the * subreg_def for this insn. Save it first. */ @@ -2970,12 +2970,6 @@ static int check_stack_read_fixed_off(st state->regs[dst_regno].live |= REG_LIVE_WRITTEN; return 0; } - for (i = 1; i < BPF_REG_SIZE; i++) { - if (stype[(slot - i) % BPF_REG_SIZE] != STACK_SPILL) { - verbose(env, "corrupted spill memory\n"); - return -EACCES; - } - }
if (dst_regno >= 0) { /* restore register state from stack */